Is anyone actually using gstreamermm?

[Carlos Rafael Giani]

On 11.03.20 21:57, Marcin Kolny wrote:
> In terms of plugins; it's actually possible to write them fully in 
> C++. Unfortunately I never managed to write any example, but there's a 
> few tests which you can treat as examples [1].


I've done that in the past. This is largely straightforward, but there 
are a few gotchas:


1. Any C++ objects in the element and element class structures must be 
initialized with placement-new (don't forget to include the <new> header 
for this!) and have their destructors explicitely called. That's because 
the structures are allocated with C allocators, which know nothing about 
constructors and destructors. Example:

struct MyGstElement
{
   GstElement parent;

   MyCppObject cpp_object;
};

....

void my_gst_element_init(MyGstElement *self)
{
   new (&self->cpp_object) MyCppObject(<constructor arguments>);
}

void my_gst_element_finalize(GObject *object)
{
   ...
   self->cpp_object.~MyCppObject();
   ...
}

Since this can get annoying quickly if you have many C++ objects in your 
element, I recommend creating one struct with all C++ objects inside, 
and then call that struct's constructor and destructor in _init() and 
_finalize().


2. Surround GST_PLUGIN_DEFINE() with extern "C" { }. Otherwise, name 
mangling may mess up the symbols that the plugin loader looks for.


3. Sometimes, people like to write internal structs and call the struct 
instances something like "private". But "private" is a C++ keyword!


4. I recommend using std::string as often as you can instead of C 
strings, since the former are _so_ much nicer and easier to use. But 
when combining them with GObject properties, GstStructures etc. you must 
always remember to convert from/to C strings. Since the type is erased 
at compile time, nothing will warn you or produce an error, you can 
easily get a crash instead.


5. C++ is more strict about implicit casts. In particular, in GParam 
functions like g_param_spec_uint(), the flags parameter is often set to 
a bitwise OR combination of enums, like G_PARAM_READWRITE | 
G_PARAM_STATIC_STRINGS . In C, this is implicitely cast to GParamFlags. 
In C++, you get a compiler error, so you must explicitely cast this, 
like so: GParamFlags(G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS)